Method for producing half-finished products made of aluminium alloys and an article manufactured with the aid of said method

ABSTRACT

The present invention relates to the method of fabrication of wrought semiproducts from high strength aluminium-based alloys, and fabrication of articles from such semiproducts e.g frames, fittings, beams, etc. used in aerospace engineering and transportation facilities. The suggested method comprises: casting an ingot, homogenization, forging the ingot, extrusion of the wrought billet, solution heat treatment, cold working in two steps: 1. stretching with 1-5% permanent set, 2. multi-axial shaping of the billet by bending while simultaneously performing the local plastic deformation of the billet by compressing (1-20%) the joint places of the blank parts being disposed at an angle of 45-135° to each other; 
 
two stage ageing: 1 st  stage=heating to 90-120° C. for 5-24 hours; 
         2 nd  stage=heating to 160-190° C. for 5-20 hours. The suggested method allows to fabricate long, irregular-shaped semiproducts from wrought aluminium-based alloys having high strength and fracture toughness and low level of residual stresses, thereby providing the fabrication of products without any distortion or buckling upon machining.

FIELD OF THE INVENTION

The present Invention relates to metallurgy, and more particularly it relates to the method of fabrication of wrought semiproducts from aluminium-based alloys and the articles made from said semiproducts. Such articles may be used in aerospace engineering, ship building, automotive and machine-building as frames, fittings, beams and other irregular-shaped long elements preferably having longitudinal and transverse ribs. In such articles the most important are: the low level of residual stresses and the related distortion and buckling during subsequent machining of semiproducts.

BACKGROUND OF THE INVENTION

Well-known is the method of fabrication of semiproducts from high strength aluminium-based alloy, which method comprising casting an ingot, homogenization, preferably extrusion of the billet, forging and hardening (solution heat treatment +quenching+aging) (U.S. Pat. No. 3.791.876 U.S. Cl. 148/2 (publ. 24.10.72)) [1]. By this method it is possible to produce irregular-shaped semiproducts having high strength but their distortion is rather great. It leads to reduction (up to 40-50%) of articles'output after machining. When producing long regular-shaped semiproducts (e.g. sheets, plates), there is widely used stretching with 1-10% permanent set in order to relieve residual stresses and to prevent distortion (Kozlovskaya V. P., Kudryashov V. G., Bunnistrov V. I. et al,. <<Technologiya Lyogkikh Splavov>>, 1976, .No. 11, pp. 16-18) [2]; (Vladimirov S. A., Vishnyakov Ya. D., Shadsky A. A. <<Technologiya Lyogkikh Splavov>>, 1977, No. 11, pp. 18-21) [3]. But this method is not applicable to complex-shaped semiproducts because stretching of such products doesn't reduce distortion to required degree owing to uncontrollable non-uniformity of deformation along stretching axis.

To relieve residual stresses and to reduce distortion of irregular-shaped semiproducts it is known to use compressing with 1-6% permanent set (Bykov L. N., Vladimirov S. A., Shadsky A. A. <<Technologiya Lyogkikh Splavov>>, 1982, No. 1, pp. 25-29) [4]. But this method also has essential restrictions because of the irregular shape of the product. In particular, the scheme of deformation by compressing is maximum effective when regular-shaped products having axial symmetry (cylindrical, circular, rectangular) are straightened.

When long articles having complex geometry (having ribs, curved elements, different thicknesses) e.g. fittings, frames, etc, are to be straightened, one can't use the method of compressing because it is not possible to provide uniformity of plastic deformation and to eliminate distortion and buckling during subsequent machining.

Well-known is the method of production of semiproducts from wrought aluminium-based alloys, which method comprising:

-   -   casting a billet,     -   homogenization of as-cast billet,     -   forging of as-cast billet,     -   extrusion of the forged billet,     -   solution heat treatment and quenching from 470° C.,     -   cold working by compressing in one direction with 1.5-5.0% set         and then in another direction normal to the first one with the         same-set,     -   aging 108° C., 7 hours+163° C., 7 hours (Japan Application No         04187747 (publ. 22.11.90) Int. Cl. C22F 1/04) [5].

The shortcoming of this method is the impossibility to produce long, irregular-shaped semiproducts with length-to-width ratio>2:1, free from quenching stresses, that means it is impossible to produce long products of high quality. Besides that, in said method relieving of residual stresses is achieved by compression with 1.5-5.0% set in one direction and then in another direction normal to the first one, with the same set, but the required change in billet shaping is not achieved. Therefore in order to provide the required shape, the subsequent step is needed which causes the additional residual stresses which in turn lead to distortion and to quality reduction during the subsequent machining.

DESCRIPTION OF INVENTION

The object of the present invention is to provide the method of fabrication of long, irregular-shaped semiproducts from wrought aluminium-based alloys with enhanced strength properties and crack resistance, and reduced level of residual stresses, which method allows to fabricate distortion free products during machining. Accordingly, there is provided method comprising:

-   -   casting an ingot,     -   homogenization of as-cast billet,     -   forging the as-cast billet,     -   extrusion of the forged billet,     -   solution heat treatment and quenching,     -   cold working in two steps:     -   the first step comprises stretching of the as-quenched extruded         billet;     -   the second step comprises multi-axial shaping of the billet by         bending while simultaneously performing the local plastic         deformation of the billet by compressing the joint places of the         blank parts, said parts being disposed at a predetermined angle         to each other,     -   ageing. Stretching is performed with 1.0-5.0% permanent set         without heating.

Multi-axial shaping of the billet by bending is performed in a die without heating, but providing local compression with 1-20% permanent set of the joint places of the blank parts (e.g. ribs and plates). Ageing of the resulted blank is performed in two stages:

-   -   1^(st) stage—heating to 90-120° C. for 5-24 hours;     -   2^(nd) stage—heating to 160-190° C. for 5-20 hours. More         preferably, method of the invention is applicable to         semiproducts consisting of the following parts: a longitudinal         rib and a plate which are disposed at an angle of 45-135° to         each other.

By suggested method it is possible to fabricate such long products as fittings, frames, beams, etc., which products have enhanced strength and crack resistance and have no distortion or buckling after machining.

The suggested method differs from prior art methods in possibility to change the blank shape after solution heat treatment and quenching by multi-axial shaping by bending in a die without heating while simultaneously relieving the residual stresses by performing the local plastic deformation by compressing (with 1-20% permanent set ) the joint places of the blank parts being disposed at an angle to each other, that is why it becomes possible to eliminate distortion and buckling during subsequent machining. Besides that, in the suggested method the blank parts, namely ribs and plates, are disposed at an angle of 45-135° to each other, and length-to-width ratio is >2:1. The above-said angle range embraces all possible structural designs of semiproducts.

Embodiments of the present invention will now be described by way of example.

EXAMPLE 1

A long fitting blank having an irregular shape: cross-shaped section of variable thickness (40-100 mm), asymmetric deflection of blank parts relative to longitudinal axis, and angle between ribs and a plate changing from 75 to 105°, was fabricated from high-strength alloy of Al—Zn—Mg—Cu system by the following method: casting of an ingot Ø 870 mm, homogenization of as-cast billet at 460° C. for 24 hours, forging the ingot at all sides at 410° C., extrusion of the cross-shaped blank at 400° C. with reduction ratio 1:10. Solution heat treatment of the extruded blank from 470° C. and water quenching at 20° C. Stretching the quenched blank with 1.9% permanent set for residual stress relieving, multi-axial shaping of the blank by bending without heating in a die while simultaneously performing the local plastic deformation by compressing (with 2% permanent set) the joint places of longitudinal ribs and plate thus finally shaping the blank, and ageing at 120° C. for 5 hours+160° C. for 15 hours.

EXAMPLE 2

From the same alloy as in Example 1, a long blank having cross-shaped section of variable thickness (30-80 mm), was fabricated. The blank had more asymmetric deflection of axis and plate as compared with Example 1, and angle between ribs and a plate changed from 45 to 135°.

In this example the following method was used: casting and homogenization of an ingot 0 650 at 460° C. for 12 hours; forging an ingot at all sides at 450° C.; extrusion of the cross-shaped blank at 390° C. with reduction ratio 1:10. Solution heat treatment of the extruded blank was performed at 470° C. with subsequent water quenching at 20° C. After quenching the blank was stretched with 2.8% permanent set. Then it was performed multi-axial shaping of the blank by bending combined with simultaneous local plastic deformation by compressing (with 20% permanent set) the joint places of blank's ribs and a plate, without heating, in a die. Ageing was performed as follows: 120° C. 24 hours+190° C. 5 hours.

EXAMPLE 3

A long beam blank with T-shaped cross-section and of variable thickness (30-65 nun), with asymmetric deflection relative to longitudinal axis, the angle between a rib and a plate changing from 60 to 110°, was fabricated from the alloy B93Π(of AL-Zn—Mg—Cu systems) according to the following schedule:

-   -   casting and homogenization of an ingot 0 650 at 450° C. for 5         hours; forging an ingot at all sides at 410° C.; extrusion of         the forged blank at 390° C. with reduction ratio 1:12. Solution         heat treatment of the extruded blank at 475° C., water quenching         at 20° C. After quenching the blank was stretched with 5%         permanent set. Then it was performed multi-axial shaping of the         blank by bending combined with simultaneous local plastic         deformation by compressing (with 10% permanent set) the joint         places of the blank's ribs and a plate, in a die without         heating. Ageing was performed as follows: 110° C. 8         hours+165° C. 12 hours.

EXAMPLE 4

From the alloy of Al—Zn—Mg—Cu system, a long cross-shaped fitting blank identical to that of Example 1, having variable (55-110 mm) cross-section thickness, was fabricated according to the method described in Japan Application No 04187747:

-   -   casting and homogenization of an ingot 0 870 at 460° C. for 24         hours; forging the as cast ingot at all sides at 390° C. with         reduction ratio 1:8. The hardening of the wrought billet         included solution heat treatment at 475° C., water quenching at         20° C. Straightening of the billet was performed by compressive         deformation with 5% permanent set in one direction and then in         another one normal to the first, with the same set.

Shaping of the final blank was performed by bending in a die without heating. Ageing was performed as follows: 110° C. 8 hours +165° C. 12 hours.

The samples for mechanical properties' and crack resistance determination were cut from final semiproducts. The rest semiproducts were machined, and the deviations of the blank's dimensions as compared to drawings' requirements, were determined. The material utilization index (MUI) was also determined.

The Table below shows the comparative properties of semiproducts obtained by the practices of the present invention and disclosed in Japan Application No 04187747.

The data given in the Table evidently show that practice of the present invention allows to fabricate long, complex-shaped semiproducts and articles from aluminium-based alloy having a pronounced high level of strength and fracture toughness (K ₁c) combined with a reduced level of residual stresses thereby providing the fabrication of products without any distortion or buckling upon machining, increase of the material utilization index by 1.5 times and avoiding of faulty production related to impermissible deviations from the dimensions pointed out in the drawing.

The described method of the invention allows to fabricate long, irregular-shaped products such as fittings, frames, beams, etc. used in aerospace engineering and transportation facilities. TABLE 1 Mechanical properties and data of efficiency of semiproducts fabrication from high strength alloy of Al—Zn—Mg—Cu system Faulty production Max. through UTS, YTS, E1, K₁c, distortion, MUI*, distortion, Example MPa MPa % MPa{square root}m mm % % 1 550 490 13 46 3.0 40 0 2 510 460 14 49 2.0 48 0 3 540 470 14 47 1.8 60 0 4 prior art 520 460 13 39 18.3 25 46 

1. Method of fabrication of semiproducts from aluminium-based alloy, which method comprising casting an ingot, homogenization, forging an ingot, extrusion of the forged billet, solution heat treatment and quenching, cold working and ageing of the billet, wherein cold working after solution heat treatment and quenching is performed in two steps, the first step of which comprises stretching of the as-quenched extruded billet, the second step comprises multi-axial shaping of the billet by bending while simultaneously performing the local plastic deformation of the billet by compressing the joint places of the blank parts, said parts being disposed at a predetermined angle to each other.
 2. Method of claim 1, characterized in that stretching is performed with 1-5% permanent set without heating.
 3. Method of claim 1, characterized in that multi-axial shaping of the billet by bending is performed in a die without heating, but providing local compression with 1-20% permanent set.
 4. Method of claim 1, characterized in that ageing is performed in two stages: the first stage comprises heating to temperature 90-120° C. for 5-24 hours, the second stage comprises heating to temperature 160-190° C. for 5-20 hours.
 5. Method of claim 1, characterized in that the blank parts are: a longitudinal rib and a plate disposed at the angle 45-135° to each other.
 6. The article made of aluminium-based alloy, characterized in that said article is fabricated by the method according to claims 1-5. 